As known, optical access networks provide end users with access to several broadband services, such as for instance Internet access, video-on-demand, telephone services, etc.
Among the known optical access networks, passive optical networks (briefly termed PON) are becoming even more widespread. Typically, a PON comprises an OLT (Optical Line Termination) and an ODN (Optical Distribution Network) connected thereto. The ODN comprises optical links and optical splitters (with typical splitting ratio of 1:32 or 1:64) arranged according to a tree topology. The tree root is connected to the OLT, while each tree branch may be terminated by a respective ONU (Optical Termination Unit), to which an end user is connected.
The OLT typically transmits downstream traffic addressed to the various ONUs in the form of optical signals having a certain downstream wavelength, while the ONUs typically transmit upstream traffic addressed to the OLT in the form of optical signals having a certain upstream wavelength different from the downstream wavelength. The downstream optical signals addressed to the various ONUs are multiplexed according to the TDM (Time Division Multiplexing) technique, while the various ONUs access the ODN for transmitting the respective upstream optical signals using a TDMA (Time Division Multiple Access) technique. Hence, downstream traffic and upstream traffic basically are in the form of a sequence of downstream frames and a sequence of upstream frames, respectively. Each frame is divided in timeslots, and each timeslot carries an optical signal addressed to or transmitted by a certain ONU.
Use of TDM/TDMA allows preventing collisions between optical signals addressed to or transmitted by the various ONUs. In order to enable a proper functioning of TDM/TDMA mechanism, the ONUs of a PON shall be subjected to an activation procedure by the OLT, before they enter their normal operational state.
In particular, the ONU activation procedure as defined by the ITU-T Recommendations G.984.3 (January 2014) sec. 10 (for GPON systems) and G.987.3 (January 2014) sec. 12 (for XG-PON systems) basically comprises three phases. During a first phase, the ONU under activation recovers the receiving clock from the OLT and synchronizes to the downstream frames. During a second phase, the ONU under activation sends to the OLT a unique identifier (e.g. its serial number) allowing the OLT to uniquely identify the ONU. During a third phase (also termed “ranging phase”), the OLT estimates the round-trip delay between itself and the ONU to be activated. This latter phase allows the OLT calculating an equalization delay to be assigned to the ONU, so as to synchronize it to the other ONUs of the PON. The second and third phases of the activation procedure are performed during a so-called “ranging window”, namely a period during which transmission of upstream optical signals from the already active ONUs is temporarily suspended. In order to prevent collisions, the duration Tranging of the ranging window shall be higher than a round-trip delay, which for a typical 20 km ODN is about 200 Hence Tranging is typically longer than the upstream frame period Tframe, which is equal to 125 μs for GPON and XG-PON systems.
EP 0 616 443 describes a ranging procedure for PONs wherein the ranging information transmitted by the substations (ONUs) to the main station (OLT) comprise a repetitive ranging-bit-pattern and, facultatively, a preamble-bit-pattern. By using ranging information with a sequence length which is eight bits less than the width of the ranging window, the received ranging information falls within the ranging window if it has a coarse ranging accuracy of ±4 bits.
Recently, multi-wavelength PONs (including also NG-PON, namely Next Generation PONs) have been proposed, which employ multiple wavelengths for upstream transmission (upstream channels) and multiple wavelengths for downstream transmission (downstream channels). For instance, the so-called NG-PON2 system defined by the ITU-T Recommendation G.989.1 (March 2013) may use several different downstream wavelengths (e.g. up to eight in the range 1596-1603 nm) and several different upstream wavelengths (e.g. up to eight in the range 1524-1544 nm). A multi-wavelength PON basically may be seen as the superimposition of multiple PONs operating at different upstream wavelengths and different downstream wavelengths over a same ODN. In particular, each PON of the multi-wavelength PON has a respective number of ONUs configured to exchange traffic with the OLT using a certain upstream wavelength and a certain downstream wavelength. All the PONs share the unique ODN using a WDM (Wavelength Division Multiplexing) technique.
Since, within each PON of a multi-wavelength PON, the communication between OLT and ONUs is based on the TDM/TDMA technique as described above, a proper activation procedure is required also for the ONUs of the multi-wavelength PON.
However, the activation procedures for PONs described above can not be straightforwardly applied in a multi-wavelength PON context. The ONUs indeed, especially in the NG-PON case, typically comprise tunable optical transmitters, namely optical transmitters which may be operated to emit on a continuous range or a discrete set of wavelengths. Such optical transmitters are conveniently not wavelength calibrated, meaning that the wavelength at which they start emitting as they are switched on is not predictable in advance with sufficient accuracy. Lack of wavelength calibration is due to the fact that calibration performed in the factory is a quite costly process, whereas the ONUs shall have a reduced cost.
Lack of wavelength calibration however may result in collisions during the activation procedure as described above. For instance, in a NG-PON2 configured with four upstream wavelengths λu1, λu2, λu3, λu4, the activation procedure of a new ONU on e.g. the upstream channel λu2 requires providing a ranging window during which upstream transmission by all the ONUs already configured on the upstream channel λu2 is suspended, while the new ONU sends to the OLT its identifier. In the meanwhile, upstream transmission by the ONUs activated on the other upstream channels λu1, λu3, λu4 continues. Hence, if the ONU under activation (which is still uncalibrated) starts emitting on a wavelength close to any of channels λu1, λu3, λu4 (e.g. λu1) it disadvantageously induces a considerable crosstalk on the upstream traffic on the upstream channel λu1. This may impair the service quality for some customers and may also cause service interruptions.
US 2013/0259482 describes an ONU registration method for TDM/WDM PONs, including transmitting from an OLT a discovery gate to the ONUs. In response to the discovery gate, an unregistered ONU transmits a register request to another OLT. A discovery window is then provided in the OLT that receives the register request, and this OLT receives the register request in the discovery window. However, also this method is not applicable when the optical transmitters of the ONUs are uncalibrated.
In order to avoid collisions, the ranging windows on the various upstream channels of a multi-wavelength PON may be synchronized. In other words, when a new ONU shall be activated on anyone of the upstream channels, ranging windows are opened simultaneously on all the upstream channels of the multi-wavelength PON, meaning that transmission of upstream traffic is suspended for all the ONUs, independently of their transmission wavelength.
EP 0 585 087 describes a ranging method for use in TDMA systems wherein the OLT continuously superimposes either a low-level, low frequency ranging signal or a low-level, high frequency ranging signal on top of the transmitted data signal that is sent from the OLT to the ONUs.
EP 0 840 963 describes a method and device for coarse ranging in a TDMA PON system, wherein a signal with a low bitrate and with a fractional optical power compared to the main informative flow is counterpropagated compared to the main informative flow.